lib: don't use strerror()

We have and provide Curl_strerror() internally for a reason: strerror()
is not necessarily thread-safe so we should always try to avoid it.

Extended ch

lib: don't use strerror()

We have and provide Curl_strerror() internally for a reason: strerror()
is not necessarily thread-safe so we should always try to avoid it.

Extended checksrc to warn for this, but feature the check disabled by
default and only enable it in lib/

Closes #7685

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